Hash table based implementation of the Map interface. This
implementation provides all of the optional map operations, and permits
null values and the null key. (The HashMap
class is roughly equivalent to Hashtable, except that it is
unsynchronized and permits nulls.) This class makes no guarantees as to
the order of the map; in particular, it does not guarantee that the order
will remain constant over time.

This implementation provides constant-time performance for the basic
operations (get and put), assuming the hash function
disperses the elements properly among the buckets. Iteration over
collection views requires time proportional to the "capacity" of the
HashMap instance (the number of buckets) plus its size (the number
of key-value mappings). Thus, it's very important not to set the initial
capacity too high (or the load factor too low) if iteration performance is
important.

An instance of HashMap has two parameters that affect its
performance: initial capacity and load factor. The
capacity is the number of buckets in the hash table, and the initial
capacity is simply the capacity at the time the hash table is created. The
load factor is a measure of how full the hash table is allowed to
get before its capacity is automatically increased. When the number of
entries in the hash table exceeds the product of the load factor and the
current capacity, the hash table is rehashed (that is, internal data
structures are rebuilt) so that the hash table has approximately twice the
number of buckets.

As a general rule, the default load factor (.75) offers a good
tradeoff between time and space costs. Higher values decrease the
space overhead but increase the lookup cost (reflected in most of
the operations of the HashMap class, including
get and put). The expected number of entries in
the map and its load factor should be taken into account when
setting its initial capacity, so as to minimize the number of
rehash operations. If the initial capacity is greater than the
maximum number of entries divided by the load factor, no rehash
operations will ever occur.

If many mappings are to be stored in a HashMap
instance, creating it with a sufficiently large capacity will allow
the mappings to be stored more efficiently than letting it perform
automatic rehashing as needed to grow the table. Note that using
many keys with the same hashCode() is a sure way to slow
down performance of any hash table. To ameliorate impact, when keys
are Comparable, this class may use comparison order among
keys to help break ties.

Note that this implementation is not synchronized.
If multiple threads access a hash map concurrently, and at least one of
the threads modifies the map structurally, it must be
synchronized externally. (A structural modification is any operation
that adds or deletes one or more mappings; merely changing the value
associated with a key that an instance already contains is not a
structural modification.) This is typically accomplished by
synchronizing on some object that naturally encapsulates the map.
If no such object exists, the map should be "wrapped" using the
Collections.synchronizedMap
method. This is best done at creation time, to prevent accidental
unsynchronized access to the map:

Map m = Collections.synchronizedMap(new HashMap(...));

The iterators returned by all of this class's "collection view methods"
are fail-fast: if the map is structurally modified at any time after
the iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a
ConcurrentModificationException. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the
future.

Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.

Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.

HashMap

HashMap

Constructs a new HashMap with the same mappings as the
specified Map. The HashMap is created with
default load factor (0.75) and an initial capacity sufficient to
hold the mappings in the specified Map.

If the remapping function returns null, the mapping is removed
(or remains absent if initially absent). If the remapping function
itself throws an (unchecked) exception, the exception is rethrown, and
the current mapping is left unchanged.

computeIfAbsent

If the specified key is not already associated with a value (or is mapped
to null), attempts to compute its value using the given mapping
function and enters it into this map unless null.

If the mapping function returns null, no mapping is recorded.
If the mapping function itself throws an (unchecked) exception, the
exception is rethrown, and no mapping is recorded. The most
common usage is to construct a new object serving as an initial
mapped value or memoized result, as in:

map.computeIfAbsent(key, k -> new Value(f(k)));

Or to implement a multi-value map, Map<K,Collection<V>>,
supporting multiple values per key:

map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

The mapping function should not modify this map during computation.

Parameters

key

K: key with which the specified value is to be associated

mappingFunction

Function: the mapping function to compute a value

Returns

V

the current (existing or computed) value associated with
the specified key, or null if the computed value is null

entrySet

Returns a Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) the results of the iteration are undefined. The set
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations. It does not support the
add or addAll operations.

forEach

Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception. Unless
otherwise specified by the implementing class, actions are performed in
the order of entry set iteration (if an iteration order is specified.)
Exceptions thrown by the action are relayed to the caller.

Parameters

action

BiConsumer: The action to be performed for each entry

get

Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key
k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)

A return value of null does not necessarily
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to null.
The containsKey operation may be used to
distinguish these two cases.

Parameters

key

Object: the key whose associated value is to be returned

Returns

V

the value to which the specified key is mapped, or
null if this map contains no mapping for the key

isEmpty

keySet

Returns a Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or addAll
operations.

merge

If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.
Otherwise, replaces the associated value with the results of the given
remapping function, or removes if the result is null. This
method may be of use when combining multiple mapped values for a key.
For example, to either create or append a String msg to a
value mapping:

map.merge(key, msg, String::concat)

If the remapping function returns null, the mapping is removed.
If the remapping function itself throws an (unchecked) exception, the
exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters

key

K: key with which the resulting value is to be associated

value

V: the non-null value to be merged with the existing value
associated with the key or, if no existing value or a null value
is associated with the key, to be associated with the key

remappingFunction

BiFunction: the remapping function to recompute a value if
present

Returns

V

the new value associated with the specified key, or null if no
value is associated with the key

putIfAbsent

If the specified key is not already associated with a value (or is mapped
to null) associates it with the given value and returns
null, else returns the current value.

Parameters

key

K: key with which the specified value is to be associated

value

V: value to be associated with the specified key

Returns

V

the previous value associated with the specified key, or
null if there was no mapping for the key.
(A null return can also indicate that the map
previously associated null with the key,
if the implementation supports null values.)

replace

Replaces the entry for the specified key only if it is
currently mapped to some value.

Parameters

key

K: key with which the specified value is associated

value

V: value to be associated with the specified key

Returns

V

the previous value associated with the specified key, or
null if there was no mapping for the key.
(A null return can also indicate that the map
previously associated null with the key,
if the implementation supports null values.)

replaceAll

Replaces each entry's value with the result of invoking the given
function on that entry until all entries have been processed or the
function throws an exception. Exceptions thrown by the function are
relayed to the caller.

Parameters

function

BiFunction: the function to apply to each entry

size

values

Returns a Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll and clear operations. It does not
support the add or addAll operations.